6. Coast Range Sediments: 50 million years of mud

(Lighter shading indicates original extent.)

After the volcanic rocks of Siletz Terrane were stuck onto the edge of North America, silt, sand, and mud began to build up on the Pacific Ocean floor off the coast of Oregon. Over tens of million of years, sea level went up and down, nearby volcanoes erupted and were eroded away, species arose and went extinct, and the two tectonic plates continued to smash into each other. Throughout this time, ocean sediments accumulated steadily and were compressed into a thick stack of sedimentary rocks. Today, many of the older rocks remain beneath the water offshore, but uplift, folding, and faulting associated with the subduction zonesee Plate Tectonics diagram have pushed up others to form much of the Coast Range. Today, sediment washed into the ocean by Oregon’s rivers continues to pile up off the coast, covering virtually all of the Oregon seafloor as far west as the Cascadia Subduction Zone.